Avoid the need for using PYTHONPATH for sudo-ing into denoise

[smarr]

I think it's a potential source for bugs to rely on PYTHONPATH and the rebench-denoise wrapper installed by pip.

We probably get more reliable results by having our own wrapper script for denoise or possibly even running it directly with the same python used for rebench.
We do know the path were we find denoise.py, and could adapt sys.path as needed reliably without modifying the environment.

This is a followup to the changes in #260.

[smarr]

It looks like this is indeed an issue in the ReBenchDB integration tests:

https://github.com/smarr/ReBenchDB/actions/runs/11124536077/job/30910272896#step:11:54

Minimizing noise with rebench-denoise failed
    possibly causing benchmark results to vary more.
    Error: Traceback (most recent call last):
  File "/home/runner/.local/bin/rebench-denoise", line 5, in <module>
    from rebench.denoise import main_func
ModuleNotFoundError: No module named 'rebench'